It is becoming ever more common to provide network booting of operating systems (OS) in enterprise environments, web server environments, and the like. Under a network operating system boot, an OS image is loaded (booted) from a network resource, such as a boot server. This scheme provides advantages relating to configuration control and generally reduces IT management costs, while at the same time reducing licensing costs.
While advantageous in many ways, the conventional network-booting scheme is unsecure. For instance, an insider may advertise the availability of a rogue boot server masquerading as a legitimate boot server that serves malicious OS images. The net result is that unknowing users load a malicious OS image, which may contain a virus that causes widespread havoc or a Trojan that sits unnoticed for days, weeks, or months until an activation event occurs, causing the Trojan code to be launched.
In view of this problem, techniques have been developed to authenticate boot images (or boot servers hosting such boot images) such that malicious or otherwise unauthentic images can be easily identified, preventing such images from being booted. For example, BOOT Integrity Services, commonly called BIS, provide a mechanism to authenticate a boot image that is derived from a DHCP (Dynamic Host Controller Protocol) offer. Even through the mechanism is sufficient to ascertain that the image is not modified in any way (i.e., is authentic), it has some shortcomings that may prevent its use in more secure environments.
One problem is the conventional scheme uses digital certificates that need to be certified. The certificate generated by the server needs to be authenticated by CA (Certificate Authority) and CRL (Certificate Revocation List) if not Self-Signed. If one of these servers is down, a false certificate may accidentally be accepted. In the case of Self-Signed certificated, its origin cannot be verified. Even though there is a provision for public key cryptography, an established mechanism for authentication of the client and boot server is still lacking. This may cause a malicious DHCP Server to act as a “Man in the Middle” or a “Malicious Proxy DHCP Server”.
Public Key cryptography can be very taxing because of exponent mathematics. This is more so in low power clients, such as PDA (Personal Digital Assistant) or wireless clients. For example, on a 16 MHz client, a 512-bit modular multiplication costs about 100 ms per calculation. As a result, decrypting a downloaded boot image via public key encryption may take seconds or even minutes, depending upon the type of client performing the download.